The scope of variables at the shell level

Question

I recently read the "Advanced" Bash Script , and I find something in the variable field between parent and child shells. so much. Here he is:

Scene: there are several ways to create a child shell: first, (command-lists) ; secondly, execute a non-built-in command or script, etc.

Because when we run the script in the parent script, the child of the script cannot see the variables in the parent shell. Why is it possible that in the structure (command-lists) child shell can see the variable in the parent shell. eg

• (team lists)

   $ a=100 $ (echo $a) 100 $ 

• run script

   $ cat b.sh echo $a $ a=100 $ ./b.sh # empty 

How?

Answer 1

In case the sub-shell runs in the initial script:

 (command1; command2; ...) 

a sub-shell is a direct copy of the original shell created by fork() , and therefore has direct access to its own copy of all the source variables available to it.

Suppose that the commands ( command1 , command2 , etc.) in the sub-shell are themselves shells. These commands are executed by the sub-shell that calls fork() and then exec() to create a new shell, and the new shell does not inherit non-exported variables from the original shell.

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Direct link to your examples:

 $ a=100 $ (echo $a) 100 $ 

Here, the sub-shell has its own copy of all variables (in particular, a ), to which the parent shell had access. Any changes made to the sub-shell will not be reflected in the parent shell, of course, like this:

 $ a=100 $ (echo $a; a=200; echo $a) 100 200 $ echo $a 100 $ 

Now your second example:

 $ cat b.sh echo $a $ a=100 $ ./b.sh $ . ./b.sh 100 $ source ./b.sh 100 $ a=200 ./b.sh 200 $ echo $a 100 $ export a $ ./b.sh 100 $ 

The variable a not exported, so when b.sh first launched b.sh it does not matter for $a , so it has something in common with an empty string. The second two examples are cheat; the shell reads b.sh script as if it were part of the current shell (no fork() ), so the variables are still available to b.sh , therefore, each time it echoes 100. (Dot or . is an older mechanism for reading script in the current shell that uses the Bourne shell in UNIX 7. The source command is borrowed from the C shells as an equivalent mechanism.)

The a=200 ./b.sh exports a throughout the command, so b.sh sees and drives off the changed value of 200 , but the main shell has a unchanged. Then, when a exported, it is available to b.sh automatically, so it sees and echoes the last 100.